1. Field of the Invention
The present invention relates to an apparatus for automatically reading a pattern original for a printed substrate in which the pattern original for the printed substrate is read optically to prepare an NC tape to be used in perforating the printed substrate.
2. Description of the Prior Art
Generally, a printed substrate for mounting electronic components includes a copper foil pattern formed on a phenol resin laminated board having a paper base, or an epoxy resin board containing glass fibers combined therewith, and a number of holes are formed in the pattern at predetermined positions for inserting the electronic components.
The holes for the electronic components may exceed 1000 even for a small substrate and, in case of a substrate for mounting highly integrated ICs, the holes are formed by drilling typically by the use of a numerically controlled automatic perforator in order to obtain the degree of perforation accuracy required.
This numerically controlled automatic perforator is arranged to perform drilling of a hole by selecting a drill of a predetermined diameter based on perforation information of an NC tape which is prepared by reading a perforation position from a pattern original made by printing a pattern of the printed substrate on a film.
In preparing the NC tape, it is general to read the coordinates of the perforation position from the pattern original. In this respect, the pattern original is formed by printing in black color a pattern of a copper foil portion used for wiring on a transparent and colorless film 50, as shown in FIG. 4 a portion for inserting an electronic component as represented by a circular pad 52, with a transparent portion 54 provided at the center thereof. This transparent portion 54 indicates a position to be perforated by the drill and also indicates a drill diameter.
In order to read the coordinates of the position of the pad on the pattern original, in the prior art, a table manually operable to move in the X and Y directions and capable of outputting coordinate information is used together with a projector disposed above the table to manually input the coordinates of the perforation position.
In other words, a portion of the pattern original positioned and placed on the table is projected with magnification onto a screen of the projector, and the center of the pad on the pattern original is aligned with a cross mark provided on the screen by manipulating the table by a manipulating handle, and X and Y position coordinate data of the table at this time is generated by manipulating a switch, and the data of the X and Y position coordinates is punched or recorded on the NC tape. This method of preparing the NC tape requires the operator to correctly align or register the center of the pad on the pattern original with the cross mark on the screen. This registration operation is cumbersome and, further, since this operation is carried out manually, a problem arises in that an inputting mistakes are possible.
In order to solve the problem of such a manual operation, as disclosed in U.S. Pat. No. 4,348,592 issued to the applicants of the present application, a pattern reading apparatus for automatically reading pads in the pattern original was proposed. In this pattern reading apparatus, a pattern original is placed on an XY table and scanned by an image sensor, such as a one-dimension photo diode array or a two-dimension photo diode array, or by an image pick-up tube, and the coordinates of the center of a hole and the hole dimeter are calculated when a change in an image signal from the image sensor satisfies a condition corresponding to a circular hole. In the case of the circular hole, the image signal changes during the scanning line in such that a white portion appears between black portions, the white portion increases gradually, and the white portion decreases gradually from a certain time point, and a solid black portion appears again.
However, in the prior art pattern reading apparatus, since the perforation diameter is read from the pattern original by the image sensor or image pick-up tube, in order to obtain accurate hoel diameter information, it is required to make the feed pitch of the XY table small, and, at the same time, to make the resolution of the image sensor or image pick-up tube higher and to increase the scanning density. Consequently, a problem arises in that the reading and processing time becomes longer. Further, in the pattern original, a diameter of a transparent portion 54 (in FIG. 4) within the pad is required to correspond to an actual drill diameter, and hence where there are many different hole diameters, a problem arises in that the preparation of the pattern original becomes cumbersome.
In order to solve the problems mentioned above, an automatic pattern original reading apparatus has been proposed in which the transparent portion in the center portion of the pad is provided with a colored indication corresponding to a hole diameter, and the hole diameter is read by detecting the color. However, in this reading apparatus, since the coloring is provided in the transparent portion of the center portion of the pad in the pattern original, and further, since the diameter of the transparent portion is usually about 0.8 mm, and a smaller diameter is as small as 0.5 mm, the color for these diameters is difficult to detect. In addition, since this apparatus is usually designed to read the coordinates of the perforation position and the hole diameter simultaneously, in order to determine the coordinates of the perforation postiion, a change in the image signal in the course of scanning, that is, a colored light portion appears between black portions of the pad, the width of the colored light portion decreases gradually on the way to ultimately resulting in a solid black portion, must satisfy the condition for a circular hole, and when this condition is satisfied it is determined that a pattern which has been read is a hole. However, at the instant when the hole to be read is determined, since the scanning position has already passed through the hole, the colored indication located at the read perforation position cannot be read anymore. Therefore, when the colored light portion appears in the black portion, the colored indication at this position must be read and stored beforehand. Further, since the stored information must be retained until it is determined whether the stored information corresponds to the hole to be read or not, a problem is involved in that the capacity of a memory device must be large and consequently data processing time becomes long resulting in much longer reading and processing time.
Furthermore, since components have recently been mounted on the printed substrate with higher density such as two pieces of wiring foil are penetrated in an interval (2.54 mm) between adjacent legs of an IC, sometimes a plurality of holes to be read happen to exist within a small breadth (e.g., 5 mm) of scanning, and hence the color detection cannot be achieved in the detection of point information if the pitch is too rough.
To avoid this drawback, it is considered to color the entire hole portion to be perforated and to distinguish it from a transparent and coloress portion in which no wiring is provided. However, if it is done in this manner, another drawback will be caused in that one of color information for discriminating drill diameters one from another will be decreased, and also a majority of holes will have to be colored one by one without leaving these holes to be transparent irrespective of the fact that actually about 80% of holes to be perforated in a printed substrate are of the same diameter.
Furthermore, in reading apparatus of the type in which the coloring in the pattern original is detected by optically scanning the surface of the pattern original, a problem exists wherein a considerable number of detecting elements corresponding to the width of the scanning must be used, and thus a capability of discriminating colors cannot be enhanced due to non-uniformity of output characteristics of individual detecting elements.